wait a minute, if we're in the same classes, and I want to stay home, and i give you my card (you now have 2, yours and mine) and I get checked in along with you. the next day, I go to class with both cards, and you stay home. Perfect attendance for both of us!
Students are devious. If you punch my ID monday, I'll punch yours Friday. (in a non-creepy way, of course)